The present invention relates to a cutter for a boring head, preferably a raise boring head (e.g., see U.S. Patent RE.34,167 disclosing a raise boring head), and in particular to a lubrication mechanism of such a cutter.
A shaft of the cutter is rotatably mounted in a saddle secured to the raise boring head. Bearings provided between the shaft and an outer roller body of the cutter make it possible for the roller body to rotate relative to the shaft and for cutting elements on the roller body to perform a cutting operation. Seals disposed between the shaft and the roller body seal-in lubrication introduced into the bearings.
When cutters of the type mentioned above are working it is necessary that the bearings are embedded in a lubricant, preferably grease, i.e., the space between the shaft and the cutter is filled with grease. This is done in order to minimize friction when the roller body is rotated relative to the shaft. By having a low internal friction in the bearings, the life of the cutter is kept at an acceptable level. However, it has turned out that under very tough working conditions, i.e., when the cutter is subjected to a heavy load, much heat is generated by rotation of the roller body relative to the shaft and by friction generated as the cutter works the rock. Due to the heat generation the grease expands and the internal pressure of the cutter rises. Under such conditions, it occasionally happens that the internal pressure of the cutter reaches a level where the seals are not able to withstand the internal pressure, i.e., the seals collapse and the grease emerges from the bearings. It is readily understood that if the seals collapse, the cutter will rapidly breakdown.
From German Patent No. 27 42 019 a cutter for a tunnel boring machine is previously known, the cutter being equipped with pressure relief valves connected to certain ends of the internal grease passages. Other ends of the internal grease passages are equipped with nipples for introducing grease into the internal grease passages. The function of the pressure relief valves is to allow passage of excessive grease both when grease is primarily introduced into the interior of the cutter as well as when regreasing of the cutter is effected. However, if the cutter according to DE-PS 27 42 019 is subjected to heavy loads and consequently a thermal expansion of the grease takes place, the pressure will increase until the pressure relief valves open. Thus, it is absolutely necessary that the valves function properly to avoid damage primarily to the grease seals and subsequently to the bearings. It is well known that the working conditions of cutters are extremely tough and that a considerable amount of dirt/cuttings adhere to the cutter. Therefore, there is considerable risk of the pressure relief valves becoming clogged and consequently not opening at their nominal pressure.